This invention relates generally to ink jet printing systems. More particularly, this invention relates to an ink jet pen with improved volumetric efficiency that maintains its performance in pressure and temperature extremes.
Ink jet pens comprise generally two components: a printhead and an ink delivery system for delivering ink to the printhead. In the manufacture of ink jet pens, various approaches have been taken to insure that a substantially constant back pressure (sub-atmospheric pressure) is provided to the printhead of the pen as the ink is depleted during an ink jet printing operation. In this manner, the size of the ink drops ejected from the orifice plate of the printhead remain constant during ink depletion. This back pressure also prevents leakage of ink from the orifice plate when an orifice is not firing.
A concern that must be addressed, however, is internal pressure changes in the pen. The internal pressure in the ink reservoir may increase relative to the ambient pressure because of high altitude transport or heating of the pen during operation. The back pressure at the printhead may thus decrease to the point that the ink can drool or even be ejected from the pen.
One approach to maintaining a substantially constant back pressure in the ink reservoir in the face of pressure changes is disclosed in U.S. Pat. No. 4,509,062 to Low et al. The described approach, while highly satisfactory and unique in most respects, nevertheless requires a collapsible bladder in order to maintain a substantially constant back pressure in the ink reservoir over a desired range of pressure and temperature. The collapsible bladder impairs the volumetric efficiency of the pen, defined as the volume of the ink jet pen divided by the volume of deliverable ink.
Another approach to storing ink in an ink reservoir is disclosed in U.S. Pat. No. 4,771,295 to Baker, assigned to the present assignee. In this approach, a reticulated polyurethane foam is placed in the ink reservoir as an ink storage medium for both black and color pens. This provides several new and useful improvements. However, the porous storage medium in the ink reservoir reduces the volume of ink therein. This approach thus offers negligible improvement in volumetric efficiency compared to the approach of Low.
To improve volumetric efficiency while maintaining substantially constant back pressure across of range of temperature and pressure, several other techniques have been tried. U.S. Pat. No. 4,794,409 to Cowger et al. describes an ink jet pen having a primary ink reservoir, a catch basin and an ink jet printhead all interconnected by a porous transfer member such as foam. As pressure within the primary ink reservoir changes relative to ambient pressure, it is intended that ink be drawn through the foam back and forth between the primary reservoir and the catch basin. U.S. Pat. No. 4,791,438 to Hanson et al. describes an ink jet pen having a primary ink reservoir, a secondary ink reservoir and a capillary member connecting the two reservoirs. The disclosed structure is also intended to draw ink through the capillary member back and forth between the primary reservoir and the secondary reservoir. Both of these devices, however, have limitations as well as advantages. The limitations stem in part from having a catch basin that is normally empty and not in fluid communication with the primary ink reservoir.